Dry cooling towers generally are the safest, but also the least effective. In dry cooling towers, heat transfers from the refrigerant to the outside, or heat transfers from the liquid refrigerant to an intermediary liquid (such as water or a glycol mixture) within a closed system, and then heat transfers from that liquid to the outside. This heat transfer is often facilitated by pumping air through the cooling tower.
Dry cooling towers operate on a similar principle as window unit air conditioners; the rear of the unit must be outside so that heat can transfer from the refrigerant to the outside. Because dry cooling towers consist of closed systems and do not necessarily contain water, legionella is generally a non-issue.